The present invention relates to a head positioning control system for use in, for example, a hard disk drive to control positioning of a head in a target position in accordance with servo data which has previously been recorded on the disk.
A disk storage system, for example, a hard disk drive (HDD) comprises a head for writing data on a disk (a storage medium) and reading data from the disk. To perform the operation for reading or writing data, a head positioning control system for controlling positioning of the head in a target position (a storage region to which an access will be made) on the disk is required.
Specifically, the target position is a specific track (a specific cylinder) on the disk of the HDD, the track (the cylinder) including a data sector to which a read access (an operation for reading data) or a write access (an operation for writing data) is made.
A HDD, as shown in FIG. 11A, has a multiplicity of tracks 100 formed concentrically on the data side of a disk 1 thereof. Servo areas 101 having servo data recorded thereon are formed at the same directional positions of the tracks 100. The servo areas 101 are formed in the circumferential direction of the disk 1 while being apart from each other at predetermined intervals. The servo area 101, as shown in FIG. 11B, is a sector (a servo sector) serving as a reference position for a plurality (for example, 50) of data sectors 104 included in one track 100. The servo area 101 has servo data mainly composed of a track code 102 and servo burst data 103 recorded thereon (when the HDD has been manufactured).
Servo data is used in an operation for controlling positioning of the head by the head positioning control system. That is, servo data is position data with which the position of the head is detected. The track code 102 is a gray code for indicating serial numbers called track addresses (or cylinder addresses) for identifying the tracks 100. The servo burst data 103 is a burst signal pattern with which a position in a range of each track 100 is detected.
When a target position on the disk, to which an access will be made, has been determined, the head positioning control system performs seek control (also called "velocity control") and track following control. The seek control is a controlling operation for moving the head to a position near a target position, the seek control being performed in accordance with a target velocity corresponding to the distance from the current position of the head to the target position. The head positioning control system detects the position of the head in accordance with the above-mentioned track code 102 read by the head to calculate the distance from the position of the head to the target position in order to determine the target velocity. Moreover, the system calculates the movement velocity in accordance with the detected position of the head so as to output an amount of a control operation (a digital value) determined in accordance with the difference between the movement velocity and the target velocity. Specifically, the amount of the control operation is a quantity for operating a head actuator for moving the head. That is, the amount of the control operation is the level of an electric current for rotating a voice coil motor (VCM) of the head actuator. The system forms a feedback control system for performing control until the target position and the current position of the head coincide with each other.
The track following control is a control operation for allowing the head to coincide with the target position. Specifically, the track following control is position control for positioning the head in a range of the target track. The system forms a feedback control system for detecting the position of the head in accordance with servo burst data 103 read from the head to perform a control operation until the position of the head and the target position coincide with each other.
When the head positioning control system has performed seek control, the head moves on a locus in the form of a curve shown in FIG. 9A. In FIG. 9A, an axis of abscissa stands for seek time which is taken for the head when a read access or a write access is made and axis of ordinate stands for positions of the head from the current position to a target position (TP). When the seek control is performed, the system outputs the amount CV of the control operation as shown in FIG. 9C.
When the seek control is performed, the head is moved in accordance with its movement velocity characteristic, as shown in FIG. 9B, composed of an acceleration mode (Ma) in an initial period, a constant velocity mode (Mc) in which maximum velocity is maintained and a final deceleration mode (Mr) after the head has approached a target position. The system is shifted from the deceleration mode (Mr) to a next operation for performing the track following control. As described above, the system executes the seek control in accordance with the target velocity. The target velocity is previously stored in a memory as target velocity data. That is, target velocity data is information in the form of a table for indicating the velocity with which the above-mentioned velocity characteristics composed of the acceleration mode (Ma), the constant speed mode (Mc) and the deceleration mode (Mr) can be obtained in accordance with the distance for which the head moves.
The seek control has been performed in such a manner that the amount CV of the control operation is adjusted in the deceleration mode in order to shorten the period of time required for the head to move to the target position. That is, as shown in FIG. 9C, the system outputs maximum amount CV of the control operation in the acceleration mode (Ma) and reduces the amount CV of the control operation in the deceleration mode (Mr) so as to adjust the amount CV of the control operation. To shorten the time required for the head to move to the target position, the head movement velocity is rapidly changed in the deceleration mode (Mr). However, the HDD encounters phenomenon called "overshoot" and "undershoot" with respect to the target position if the head is, adjacent to the target position, rapidly decelerated because of an error of its mechanical system or dispersion in the characteristics of the circuit system occurring owning to the tolerance in the manufacturing process. As a result, the head is caused to considerably deviate from the target position. Therefore, even if the time required for the head to move to a position near the target position is shortened, time (time to perform the track following control) required for the head to be made coincide with an allowable range in which data can be written or an allowable range in which data can be read is elongated undesirably. Thus, the read seek time (time for which the head performs seeking when a read access is made) and the write seek time (time for which the head performs seeking when a write access is made) are elongated.
If the system rapidly changes the head movement velocity in the deceleration mode during the seek control operation, the probability of occurrence of the overshoot phenomenon, which is a phenomenon for the head to pass the target position, and the undershoot phenomenon, with which the head cannot reach the target position, is increased. Accordingly, excessive deviation of the head from the allowable range in which data can be written or the allowable range in which data can be read is prevented by moderately setting the decelerating locus on which the head is moved in the deceleration mode. That is, stabilization for accurately moving the head to a position near the target position is given priority to shortening the seek time for the head.
The head must be positioned in a target position at different accuracy level between the read access and the write access. That is, as shown in FIG. 10A, an allowable range 201, into which the head must be positioned when the write access is made, is determined to be narrower than an allowable range 202 into which the head must be positioned when the read access is made if the range (the range for the target track) for the target position is the same. FIG. 10A shows a movement locus 200 for the head when the write seek control is performed. If the head reaches the range of the adjacent track when the head writes data on the range of the target track to perform a data writing operation by making a write access, the possibility of breakage of data recorded on the adjacent track is enlarged. Therefore, the allowable range 201 into which the head must be positioned is required to be relatively narrow when the write access is made. That is, positioning control more accurate than that required in the read access operation must be performed.
However, the conventional head positioning control system has been arranged to perform the seek control with the same velocity characteristic in both of the cases where the read access is made and the write access is made. That is, the feedback control system for the seek control has been designed by using the accuracy as a reference which is required when the write access (the operation for writing data) is made in which the allowable range into which the head must be positioned is narrow. Therefore, when the read access is made in which a wide allowable range 202 is permitted, the head moves on a movement locus 203 with remainder capacity as compared with the performance of the system for positioning the head, as shown in FIG. 10B. When the read access is made, the allowable accuracy range 202 for the head is relatively wide. Therefore, the performance of the system may be used to shorten the head seek time as compared with the positioning accuracy. Since the accuracy allowable range 201 for positioning the head is relatively narrow when the write access is performed, the performance of the system must be used to improve the positioning accuracy.